igneous

Basalt

Basalt (Mainly Plagioclase Feldspar and Pyroxene)

Hardness: 6 (Mohs scale); Color: Dark gray to black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); Cleavage: None (conchoidal fracture); Specific gravity: 2.8–3.0

Hardness: 6 (Mohs scale)
Color: Dark gray to black
Luster: Dull to sub-metallic

Identified March 11, 2026More igneous →

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Physical properties

Hardness: 6 (Mohs scale); Color: Dark gray to black; Luster: Dull to sub-metallic; Crystal structure: Aphanitic (fine-grained); Cleavage: None (conchoidal fracture); Specific gravity: 2.8–3.0

Formation & geological history

Formed from the rapid cooling of magnesium-rich and iron-rich lava exposed at or very near the surface of a terrestrial planet or a moon. It is the most common rock type in the Earth's crust (oceanic crust).

Uses & applications

Used primarily in construction for aggregate, road base, railroad ballast, and concrete. It is also used as dimension stone for statues and architecture, and crushed for industrial high-temperature insulation.

Geological facts

Basalt forms spectacular columnar joints (like the Giant's Causeway) as it cools and contracts. It is also the most common rock on the Moon's surface, making up the dark 'Maria' plains visible from Earth.

Field identification & locations

Identify in the field by its dark color, heavy weight (density), and fine-grained texture with no visible crystals to the naked eye. Commonly found in volcanic regions, riverbeds, and as garden landscaping stones.

Other igneous specimens

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Blue Apatite

Fluorapatite (Ca5(PO4)3F)

mineral

Dark Igneous Rock (Likely Basalt or Diabase)

Basalt (extrusive igneous rock) or Diabase (intrusive igneous rock - also known as Dolerite), largely composed of plagioclase feldspar, pyroxene (augite), and sometimes olivine and amphibole. Exact mineralogy would require thin section analysis.

Igneous

Magnetite

Iron(II,III) oxide (Fe3O4)

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Blue Apatite

Apatite (Group), Ca5(PO4)3(F,Cl,OH)

mineral

Magnetite

Magnetite (Fe3O4)

mineral